The present invention generally relates to image forming apparatuses, and more particularly to an image forming apparatus which forms an image on a recording sheet such as a paper by an electrophotography method.
FIG. 1 shows an essential part of an example of a conventional image forming apparatus which forms an image on a paper by an electrophotography method. The image is written on a photosensitive drum by a laser beam which scans the photosensitive drum, and an electrostatic image is formed on the photosensitive drum by this scanning of the laser beam. The electrostatic image is developed into a visible image, and the visible image is transferred on the paper and fixed thereby completing an operation of forming the image on the paper.
In FIG. 1, a photosensitive drum 10 is provided rotatably in a direction of an arrow. A charger 12, a developing unit 16, a transfer/separation charger 18, a discharger 20, a cleaning unit 22 and a discharge lamp 24 are provided in a vicinity of the photosensitive drum 10 in this sequence along the rotating direction of the photosensitive drum 10.
An optical scanning unit 14 optically scans the photosensitive drum 10 by a laser beam L the light intensity of which has been modulated depending on an image signal describing an original image. The laser beam L scans the photosensitive drum 10 between the charger 12 and the developing unit 16 so as to write the image on the photosensitive drum 10.
The image forming apparatus is also provided with cassettes 36 and 38 which accommodate recording paper S, feeding units 26 and 28 for feeding the paper S from the respective cassettes 36 and 38, resist rollers 30, a fixing unit 32, and a tray 34 for receiving the paper S having the image formed thereon.
The image is formed on the paper S as follows. First, the photosensitive drum 10 starts to rotate clockwise, and a surface of the photosensitive drum 10 is discharged by the discharger 20. Then, the surface of the photosensitive drum 10 is uniformly charged by the charger 12. The image is written on the charged surface of the photosensitive drum 10 by the scanning of the laser beam L emitted from the optical scanning unit 14, and an electrostatic image is formed on the surface of the photosensitive drum 10. This electrostatic image corresponds to the image to be formed on the paper S. The electrostatic image is developed by the developing unit 16 and becomes a visible image (toner image).
On the other hand, during this time, the paper S accommodated within the cassette 36, for example, is fed by the feeding unit 26 and the paper S is set to a waiting position where a tip end of the paper S is pinched between the resist rollers 30. The resist rollers 30 supply the paper S in the waiting position to a transfer part of the image forming apparatus in synchronism with a movement of the visible image on the photosensitive drum 10.
In the transfer part, the visible image on the photosensitive drum 10 is transferred onto the paper S by the transfer/separation charger 18, and the paper S having the visible image thereon is separated from the photosensitive drum 10 and transported towards the fixing unit 32. The visible image on the paper S is fixed by the fixing unit 32, and the paper S having the fixed visible image thereon is ejected onto the tray 34.
After the visible image is transferred onto the paper S, the surface of the photosensitive drum 10 is cleaned by the cleaning unit 22 so as to remove residual toner on the photosensitive drum 10, and the surface of the photosensitive drum 10 is discharged by the discharge lamp 24.
The image is written on the photosensitive drum 10 by the laser beam L so that a portion of the surface of the photosensitive drum 10 corresponding to a white portion of the image is exposed and a portion of the surface of the photosensitive drum 10 corresponding to a black portion of the image is not exposed but visualized by the toner. When forming the image on a plurality of paper S, the supply of the paper S is successively carried out from a portion A shown in FIG. 1.
When the paper S having the image formed thereon by the conventional image forming apparatus is examined in detail, it is found that the density of some black portions of the recorded image is relatively lower than the density of other black portions of the recorded image, even though the black portions should originally have the same density. In other words, even when all of the black portions of the original image have the same density, some black portions of the recorded image on the paper S have a density relatively lower than that of other black portions of the recorded image. The generation of the black portions of the recorded image having the relatively low density is especially conspicuous at a boundary portion between a black portion and a white portion, and also when the original image consists of general characters or a pattern of fine lines. In addition, it is found that the density of the black portion of the recorded image becomes low especially in a vicinity of both end portions of each scan of the laser beam L along a main scanning direction.
Therefore, there is a problem in that the picture quality of the recorded image is unsatisfactory due to the inconsistent density of the black portions of the recorded image.